A mill or treatment plant for metal strip, normally steel, is typically provided with a system for circulating loaded pallets and having multiple roller conveyors. The product, typically a coil of metal strip, is carried on a pallet having two spaced longitudinal running rails and a coil-holding saddle bridging them and forming confronting support faces. The coil is set in the saddle and is transported from the production line where it is rolled and formed into coils to the downstream treatment stations where it may be, for instance, heat treated. The coils are set on and lifted off the pallets, and the empty pallets are recirculated back upstream to the rolling mill for reuse.
Such pallet-circulating systems have been used for a long time for cold coils, that is for coils at the end of a rolling stream producing cold metallic strip, e.g. of aluminum. They can move coils weighing as much as 45 ton and can be quickly recirculated. The advantage of such a system is that the pallets, even though of substantial construction, are still fairly light compared to the massive loads they carry, so that they are easy to move about and position. These pallets are also easy to service, inexpensive to manufacture, and easy to use in general. They allow extremely cumbersome loads that can themselves vary considerably in size and shape to be moved about as if they were all the same.
Such a circulating system using pallets is not suitable for conveying heavy, for example 30 to 40 ton, bundles or coils that are very hot, that is with a surface temperature of the coil between 500° C. and 800° C. In addition to the considerable weight, the considerable heat of the loads being transported causes the pallets to deform substantially, often to the extent of being several millimeters out of shape. The deformations are not uniform, since some parts of the pallets, which are always made of metal, are closer to the hot load or are more directly exposed to the radiant heat emitted by it, so that there is considerable differential expansion.
The main problem is that the lower surface of the pallet defined by its running rails is deformed out of a planar shape. Since these rails ride on arrays of rollers that are all tangent to a normally perfectly horizontal conveyor plane, this means that the pallets do not ride smoothly. Instead of sitting on a plurality of the rollers at any given instant, they sit on only one or two, and bump down the line. This quickly damages the pallet and, when it is carrying a freshly rolled and still very hot coil, the coil itself can be ovalized and made difficult to subsequently unroll and handle. In addition the rollers are subjected to considerable stress, in particular when struck by a corner of an end of a running rail.